Caption: Avian influenza (flu) virus. Computer artwork of H5N1 avian (bird) influenza virus particles. Wild birds, the virus' natural hosts, show few symptoms when infected but domestic birds suffer a 90-100 per cent mortality rate. It can be spread to humans by inhaling faeces particles from infected birds, and can be fatal. It is feared it could mutate into a form able to be transmitted from human to human, potentially leading to hundreds of thousands of deaths.